In less than a week this month, the stomach-turning epithet for ammonia-treated ground beef filler suddenly became a potent rallying cry by activists fighting to ban the product from supermarket shelves and school lunch trays. The U.S. Department of Agriculture is set to announce today that it will offer schools choice in ground beef purchases in response to requests from districts.

Though the term has been used pejoratively for at least several years, it wasn't until last week that social media sites suddenly exploded with worry and an online petition seeking its ouster from schools lit up, quickly garnering hundreds of thousands of supporters.

The controversy centers on "lean finely textured beef," a low-cost ingredient in ground beef made from fatty bits of meat left over from other cuts. The bits are heated to about 100 degrees and spun to remove most of the fat. The lean mix then is compressed into blocks for use in ground meat. The product, made by South Dakota-based Beef Products Inc., also is exposed to "a puff of ammonium hydroxide gas" to kill bacteria, such as E. coli and salmonella.

There are no precise numbers on how prevalent the product is, and it does not have to be labeled as an ingredient. One industry official estimates it is in at least half of the ground meat and burgers in the United States. Past estimates have ranged as high as 70 percent.

It has been on the market for years, and federal regulators say it meets standards for food safety.